add help docs and ?

docs/nodes/Entropy.md

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+# Entropy
+
+Compute the Shannon entropy of the height or slope distribution. H = −Σ p·ln(p). Equivalent to Gwyddion entropy.c.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| entropy | FLOAT | Shannon entropy in nats |
+| normalised_entropy | FLOAT | Entropy divided by ln(n_bins), in [0, 1] |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| mode | dropdown | height values | Compute entropy of height values or slope magnitude |
+| n_bins | INT | 256 | Number of histogram bins for probability estimation (16–1024) |
+
+## Limitations
+
+- Entropy is sensitive to n_bins; very few bins underestimate entropy while very many bins overestimate it for small fields.
+- Non-finite pixel values are removed before binning.